1. Field of the Invention
The present invention relates to an image sensing apparatus applied to interval photography which repeatedly performs photography (obtains still images) at predetermined time intervals.
2. Description of the Related Art
In microscopy, there is available a method of observing a change in sample over time while performing interval photography at predetermined photographing intervals using an image sensing apparatus, i.e., an observation method called time-lapse photography. This observation often uses fluorescence microscopy. Since light emitted from a sample is too weak in photographing the sample by a fluorescence microscope, the exposure time for an image sensing apparatus needs to be prolonged.
In an image sensing element such as a CCD or the like which is conventionally used in an image sensing apparatus, a current called a dark current may be generated even when the image sensing element is not receiving light. This dark current exerts a greater influence as the storage time of optical signal charges in the image sensing element becomes longer. Also, the dark current has high temperature dependence. As for the temperature dependence, when the temperature increases by about 8 to 10° C., the value for the dark current roughly doubles.
If an image sensing apparatus performs photography with a longer exposure time, dark noise may occur in an image in fixed patterns because of a dark current.
As a general method of temporarily reducing dark noise, there is available the following method.
An object is first photographed normally using a light-shielding shutter in an open state. Then, the light-shielding shutter is closed, and an image sensing element is set in a light-shielded state. A light-shielded image is photographed with the same exposure time as that for the normal object photography. The light-shielded image is subtracted from the normally photographed image, thereby obtaining an image with suppressed dark noise.